Experimental study on the effects of channel gap size on mixed convection heat transfer characteristics in vertical rectangular channels heated from both sides

The effects of channel gap size on mixed forced and free convective heat transfer characteristics were experimentally investigated for water flowing near atmospheric pressure in a 750 mm long and 50 mm wide channel heated from both sides. The channel gap sizes investigated were 2.5, 6, 18 and 50 mm. Experiments were carried out for both aiding and opposing forced convective flows with a Reynolds number Rex of 4x10 to 6x106 and a Grashof number Grx of 2x104 to 6x1011, where the distance x from the inlet of the channel is adopted as the characteristic length in Rex and Grx. As for the results, the following were revealed for the parameters ranges investigated in this study. (1) When the dimensionless parameter, Grx/Rex21/8Pr1/2 is less than 10-4, the flow shows the nature of forced convective heat transfer for a channel with any channel gap size in both aiding and opposing flows. (2) When Grx/Rex21/8Pr1/2 is larger than 10-2, the flow shows the nature of free convective heat transfer for a channel with any channel gap size in both aiding and opposing flows. (3) When Grx/Rex21/8Pr1/2 is between 10-4 and 10-2 for the channel with a channel gap size equal to or larger than 6 mm, the heat transfer coefficients in both aiding and opposing flows become, on the average, higher than those predicted by the previous correlations for either the pure turbulent forced convection or the pure free convection, and can be expressed in simple forms with a combination of Grx/Rex21/8Pr1/2 and the previous correlation for either the pure turbulent forced convection or the free convection along a flat plate. (4) When Grx/Rex21/8Pr1/2 is between 10-4 and 10-2 for the channel with a channel gap size of 2.5 mm, the heat transfer coefficients in both aiding and opposing flows also become, on the average, higher than those predicted by the previous correlations for either the pure turbulent forced convection or the pure free convection. (orig./GL)